The present invention relates to amplifier circuits, and more particularly to a CCD gate driver circuit that drives the gate capacitance of a CCD at high frequencies between two arbitrary voltage limits while requiring low bias current from the circuit power supply after a transition.
In driving the gates of a CCD (Charge Coupled Device) device it is necessary to convert an ECL or TTL clock signal to appropriate voltage limit levels compatible with the CCD device. The voltage limit levels may encompass ground potential, i.e., +8 to -3 volts, or may be completely above ground potential, i.e., +8 to +1 volts. Therefore a gate driver for a CCD device needs flexibility. In order to achieve such flexibility the logic input signal and circuit power supplies need to be independent, i.e., have different reference levels. Also, especially for high frequency applications, a symmetrical gate drive signal is desired with rapid transition from one voltage limit level to the other. Another desirable feature is to have low power usage, especially for surface mount circuitry, to avoid excessive heating of the active circuit components. Finally the voltage output level may be affected by gate to gate overlap capacitance in the CCD device, so that such effect needs to be compensated.
One typical gate driver is the DS0026 gate driver, manufactured by National Semiconductor of Santa Clara, Calif. However the input and output are tied to the same power supply levels. Also the output provides an asymmetrical output waveform which is not desirable for high frequency applications. Another CCD driver is the TSC430, manufactured by Teledyne Components of Mountain View, Calif. This driver has the input and output power supplies decoupled, but uses complementary FETs so that during transition there is a crossover current through both FETs. At high frequencies these FETs get hot and use a considerable amount of power. Also the rise time at the output is a function of the "ON" impedance of the FETs. A proprietary CCD driver circuit, developed by Tektronix, Inc. of Beaverton, Oreg., uses an output current source truncated at the voltage limit level. However this circuit is not convenient for TTL logic inputs, and current flows constantly between the voltage rails so that the circuit generates excessive heat.
Therefore what is desired is a CCD gate driver that provides a current output between two voltage limit levels at high frequencies without generating excessive heat while still providing a symmetrical output with the flexibility to accommodate various output voltage limit levels.